A Study of Magnetic and Spectroscopic Properties of Fe(II)Fe(III)2O(CH3CO2)6L3, L=H2O or C5H5N. Direct Observation of the Thermal Barrier to Electron Transfer in a Mixed-Valence Complex

摘要

The mixed-valence iron (II, III, III) acetates, (Fe3O(CH3CO2)6L3), where L= water or pyridine, have been prepared and studied by Mossbauer, infrared, and optical spectroscopy and magnetic susceptibility methods. Variable-temperature magnetic susceptibility data for the aquo complex are interpreted on the basis of HDVV S2 = 2, S1 = S3 = 5/2 spin exchange model with J12 = j23 = 1/-50 cm and J13 = 1/14.5 cm. An intervalence transfer band is observed at 1/138000 cm in the room temperature electronic spectrum of the aquo complex. Mossbauer spectra of these compounds are markedly temperature dependent. At 17K absorptions due to distinct Fe(II) and Fe(III) sites are observed while at 300 K a single absorption is observed. Spectra at intermediate temperatures are modeled by assuming intratrimer electronic relaxation between pairs of iron ions. The activation energy for relaxation derived from this model is 1/470 cm for the aquo complex. This energy is equal to the barrier to intramolecular, intervalence thermal electron transfer. (Author)